1. Field of the Invention
The present invention relates to a communication apparatus, and more particularly to a communication apparatus which automatically responds in response to a call signal supplied from a line connected to the apparatus.
2. Related Background Art
In a prior art communication apparatus such as a facsimile machine, an apparatus having an automatic receiving function which detects a call signal sent from an exchange and automatically starts data reception has been known.
FIG. 5 shows a detection process for a call signal in the prior art. Numeral 41 denotes an original waveform of a call signal which comprises an on-off tone signal of a selected frequency.
In FIG. 5, when the presence or absence of the call signal is detected, a rectangular pulse 42 is generated by detecting a curvature point of an input signal, and the rectangular wave is integrated to produce a waveform 43, which is compared with a predetermined threshold TH to produce a call signal detection signal CI having a waveform 44.
Data on the lengths of the on-period (low level period because of a negative logic) T1 and the off-period T2 of the detection signal CI and a repetition frequency thereof has been previously stored in a memory for the predetermined call signal A control unit of the apparatus determines the reception of the call signal only when the detection signal CI having those parameters is produced. In a simpler method, the call signal is detected when the call signal CI is on for more than a predetermined period.
The prior art call signal detection system is sufficiently effective when only one type of call signal is used. However, when a line and an apparatus are connected through a private branch exchange (PBX), the PBX produces different call signals for a call from an extension line and a call from an external line (public line) but the prior art apparatus cannot discriminate the difference in the call signals.
Namely, in the prior art communication apparatus, the parameters such as the on/off time and the repetition frequency of the call signal detection signal CI are fixed. Thus, if those parameters are fit to the waveform of the call signal from the external line generated by the PBX, the automatic response to the call from the extension line is not attained. Conversely, if they are fit to the waveform of the call signal from the extension line, the automatic response to the call from the external line is not attained.
The techniques to respond by the detection of call are disclosed in the U.S. Pat. Nos. 4,677,660, 4,823,375, 4,815,121, 4,800,439, 4,843,479, 4,837,806 4,663,788, 4,916,607 and 4,932,048.